Incidentally Dan note that my original question was not about hosting asterisk and not about the NAT of IPs, it was about phones and rewriting the SIP header, which causes issues with our hosted platform. As far as I understand from the docs, Static port is just releated to keeping the ports the same during NAT, which I made clear at the top was not an issue (as far as I can tell) - NAT has never been an issue for us and basic PAT (which is what cable/DSL modems do by default) changing source ports etc works absolutely fine.
I believe I understand NAT and PAT quite well on a general basis as I am CCNP and my day job is firewall based (but Cisco, Checkpoint etc and I can't get them to move to pfsense, though I try!) and I don't believe it to be a problem (unlike most SIP problems it seems!)